Disc drives of the type known as "Winchester" disc drives are well known in the industry. Such disc drives record digital data on a plurality of circular, concentric data tracks on the surfaces of one or more rigid discs. The discs are typically mounted for rotation on the hub of a spindle motor. In disc drives of the current generation, the spindle motor rotates the discs at speeds of up to 10,000 RPM.
Data are recorded to and retrieved from the discs by an array of vertically aligned read/write head assemblies which are controllably moved from track to track by an actuator assembly. The read/write head assemblies typically have an electromagnetic transducer carried on an air bearing slider. This slider acts in a cooperative hydrodynamic relationship with a thin layer of air dragged along by the spinning discs to fly the read/write head assembly in a closely spaced relationship to the disc surface. In order to maintain the proper flying relationship between the read/write head assemblies and the discs, the read/write head assemblies are attached to and supported by head suspensions.
The actuator assembly used to move the read/write heads from track to track has assumed many forms historically, with most disc drives of the current generation incorporating an actuator of the type referred to as a rotary voice coil actuator. A typical rotary voice coil actuator consists of a pivot shaft fixedly attached to the disc drive housing base member closely adjacent the outer diameter of the discs. The pivot shaft is mounted such that its central axis is normal to the plane of rotation of the discs. An actuator housing is pivotally mounted to the pivot shaft and supports a coil which is suspended in a magnetic field of an array of permanent magnets. Opposite the coil, the actuator housing also typically includes a plurality of vertically aligned, radially extending actuator head mounting arms, to which the head suspensions are mounted. When controlled direct current is applied to the coil, an electromagnetic field is formed which interacts with the magnetic field of the permanent magnets to rotate the actuator housing in accordance with the well-known Lorentz relationship. As the actuator housing rotates, the read/write heads are moved radially across the data tracks.
Control of the movement of the read/write heads from track to track on the disc surfaces is commonly accomplished through the use of a closed-loop servo system. Such servo systems typically make use of servo information recorded on the disc surfaces at the time of manufacture to obtain general information defining the specific track number and the sector position of the disc relative to the read/write head. When an access command is sent to the disc drive, a comparison is internally made between the current position of the read/write heads relative to the disc and the location at which the desired data transfer is to take place. If the read/write heads are currently positioned over the desired tracks, the disc drive simply waits for the correct circumferential location to rotate under the read/write heads, and begins the requested data transfer. If, however, the data transfer is to take place at a location other than the current position of the actuator, the servo logic determines both the distance and direction in which the actuator must move in order to bring the read/write heads to the target track. Based on this determination, the servo logic applies controlled direct current to the coil of the actuator voice coil motor (VCM), which causes the actuator to move from the current track location to the target track.
During such seek operations, the servo logic monitors the dynamic position of the actuator by reading the prerecorded servo data from the disc surfaces during the seek, and controls the current applied to the VCM in a manner to bring the read/write heads to rest at the target track.
A second function of the servo system is to maintain the read/write heads over the centerline of the target track, so that data transfers can be accomplished without inadvertently accessing adjacent tracks. This "track following" function is accomplished by constantly monitoring a position error signal (PES) which is proportional to the relationship of the read/write heads to the track centerline. That is, when the read/write heads are perfectly centered on the data track, the PES is zero, and no current is applied to the actuator VCM. Any tendency of the read/write heads to move away from the track centerline results in the generation of a PES with a polarity reflective of the direction in which the read/write head is displaced from the track centerline. The PES is then used by the servo system logic to generate a correction signal to move the read/write heads back toward the track centerline until such time as the PES is again zero, indicating that the read/write heads are again properly aligned with the data tracks.
Disc drives of the current generation typically provide maximum track densities of the order of ten thousand to twenty thousand tracks per inch measured radially across the disc, and future generations of disc drives are expected to continually increase the density. In current disc drives, the magnitude of the allowable mispositioning envelope between read/write head position and track centerline is on the order of approximately 10 microinches (0.000010 inches) peak to peak. With future track densities ever increasing, the servo system of the disc drive must be capable of controlling the position of the actuator with ever-increasing accuracy, and the allowable mispositioning envelope must be proportionally reduced.
Typically, the actuator is supported about the pivot shaft by a number of ball bearings having an outer race that rolls smoothly about an inner race due to a plurality of balls interposed therebetween. A geometrical analysis of current disc drives reveals that a tracking envelope of approximately 10 microinches at the read/write head and disc interface imparts a motion to a ball within the ball bearing of only a few tenths of a microinch at the contact points between the balls and the bearing races. Since it is common practice to preload the ball bearings in order to provide necessary stiffness of the actuator pivot, ball motions of this order are much smaller in scale than the size of the localized elastic deformations at the ball/race interface caused by the preload force. Typically, the elliptical contact area between the balls and the race is on the order of several hundred microinches. The end result is that on the microinch and sub-microinch scale, i.e., for extremely small peak-to-peak actuator motions, the balls do not rotate during a servo track following mode. Instead, the balls simply have a shifting a stress distribution, that is, the flattened, elliptical shape shifts back and forth.
This elastic deformation behavior exhibits itself in the servo system as a relatively stiff torsional spring force coupled to the actuator inertia. Force on the ball and displacement are essentially in phase during cyclical motion of the actuator. If, however, the peak-to-peak actuator motions get slightly larger, then the balls will tend to begin actually rolling back and forth. With such a tendency to roll, there is a reduction of the contribution of ball deformation to the apparent stiffness. At the same time, however, race deformations ahead of the rolling elements begin to participate in the dynamics of the actuator. The elastic strain energy required for the balls to roll is only partly recovered from the rebounding race material behind the rolling element, and the rest is dissipated as heat. The result is that ball displacement response becomes out of phase with the force trying to move the ball. This effect is referred to as hysteresis, and depends, to a large extent, on the damping properties of the specific materials of the bearing components.
In addition to hysteresis, the ball/race interface of a ball bearing experiences two other kinds of rolling resistance: 1) spinning friction, related to the fact that the contact area on the ball tends to rotate relative to the contact area on the bearing race at the start of motion (essentially a contact area torsional friction effect); and 2) frictional resistance, related to the fact that distances from the rotary axis of the ball to individual points in the contact area vary, as do the associated peripheral speeds, causing the middle section of the ball contact area to slide opposite to the direction of rolling while the outer sections slide in the rolling direction.
Generally, all of the components offering resistance to movement of the actuator are combined under the category of rolling contact friction. One of skill in the art will realize that the phenomenon is extremely complex and difficult to analyze. What is known, however, is that hysteresis and spinning friction are dependent on individual ball load, Q, to the four-thirds power (Q.sup.4/3). Sliding friction is dependent on individual ball load, Q, to the five-thirds power (Q.sup.5/3) (see Dutrowski, R., Energy Losses of Balls Rolling on Plates (ASME Transactions, Series D, J. Basic Eng. 81 (1959), 2, 233-238 and Tabor, D., The Mechanism of Free Rolling Friction (Lubrication Engineering, November-December (1956), 379-386). The obvious conclusion is that, if one wishes to reduce rolling contact frictional losses, the key is to reduce Q. Reducing the magnitude of Q through preload reduction must be done judiciously, however, since overall bearing axial and radial rigidity is also dependent on the one-third power of Q (Q.sup.1/3).
With the small motions of disc drive actuators typical to servo track following, experiments have shown that hysteresis is the major contributor to performance. Mathematically, hysteresis can be characterized as a complex stiffness that has real and imaginary parts. Note that hysteresis is different from viscous damping inasmuch as hysteresis does not depend on the frequency of oscillation of the system. In the case of ball bearings, hysteresis is rather totally dependent on the peak-to-peak magnitude of the displacement. It is this dependency on magnitude that makes hysteresis a particularly difficult problem for servo systems engaged in track following mode activity.
In efforts to eliminate or minimize the effects of ball bearing hysteresis in disc drive actuators, other types of pivot mechanisms have been proposed for disc drive actuators. Such proposed alternatives include fluid bushings, jewel bearings, spring-loaded line-contact pivots, knife edges, etc. Ball bearings incorporating ceramic balls and races have also been explored. Although hysteresis reduction has been achieved in some of these alternative pivot mechanisms, typically the gains in hysteresis reduction have been accomplished by a significant loss of simplicity in the actuator design, a loss of necessary pivot rigidity or some other undesirable characteristic.
Therefore, a need clearly exists for a pivot mechanism for the actuator in a disc drive that provides lowered hysteresis while still maintaining other desirable characteristics.